If articles are to be produced in very high numbers, then a known "multi-cavity" moulding system employing current mono-material hot-runner distribution principles may be used. In such an arrangement, the output stream from each extruder (plasticiser) is fed by a respective injection unit under pressure, through a hot runner distribution system in which the stream is split so as to serve a plurality of co-injection nozzles. In such a multi-cavity system the exit point for each layer material at the nozzle needs to be separately valved. See Patent Publications EP 0125787 A2 (Kudert et al).
Such an arrangement results in extreme mechanical complexity. Moreover, when operating in a production line, such equipment requires long set-up times, and incurs high maintenance costs, all of which adversely affect the economics of the manufacturing operation.
A further disadvantage is tha the engineering design of the runner system erquires to be optimised for a specific number of cavities at a specific product size, and thus for a specific cavity spacing.
It is also conventional to cool the article in the mould, and then to remove it only after cooling.